Combination limit switch



Feb. 9, 1937. L. M. PERSONS 2,070,175,

COMBINATION LIMIT SWITCH Filed April 29, 1955 CURRENT SUPPLY five/71W[/4 lure we Z farm/z: Jaw

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Patented Feb. 9, 1937 NITED STATES PATENT OFFICE 2,070,175 COMBINATIONLIMIT swrrcn Application April 29, 1935, Serial No. 18,740

2 Claims.

The object of my invention is to provide a combination limit switchhaving a single switch therein for opening and closing the circuit of afuel burner or the like, with a thermostatic element responsive to thetemperature of the fuel burner and operating the switch as a limitswitch, other actuators also being provided for the one switch.

Still a further object is to provide a combination limit switch having asingle set of switch contacts for controlling the motor of a fuelburner, so that only one set of contacts need be examined when the motorcircuit does not properly flow through the switch, this set of contactsbeing operated by a limit control whereby, by installing the combinationswitch, a safe installation is insured because the switch is primarily alimit switch and installed as such, whereas in the type of installationwhere a limit switch is to be installed independent of a motor or relayswitch, the dealer in order to cut down expense sometimes eliminates thelimit switch and that is not possible with my arrangement.

A further object is to provide the switch contacts actuated also by awire motor, which in turn is controlled by a room thermostat, wherebythe motor switch is not immediately responsive to the room thermostat,thus preventing undesired making and breaking of the motor circuit whenthe contacts of theroom thermostat are jiggled from any cause such asvibration of the wall and even vibrations of the atmosphere caused bytrucks or the like passing the house.

A still further object is to provide a limit switch having a protrudingtemperature responsive element for mounting in the bonnet of a furnace,and so arranged that wiring is simplified, it merely being necessary torun current wires to the switch and from the switch to the motor, alloperating parts for the switch being included within a casing therefor,the switch necessitating but two controls, a room thermostat and a stackswitch which can be operated on a low voltage circuit, thus reducingwiring expense.

A still further object is to provide the wire motor with a compensationfeature so that under all conditions it operates in the same period oftime, the present application being a continuation in part of myco-pendlng application Serial Number 638,114, filed October 17, 1932.

With these and other objects in view my invention consists in theconstruction, arrangement and combination of the various parts of thedevice, whereby the objects contemplated are attained, as hereinaftermore fully set forth,

Figure 3 is a sectional view on the line 3-3 v of Figure 1; and

Figure 4 is an electro, diagrammatical view 10 showing the wiringconnectionswith the motor to be controlled and with the controllingdevices for the switch.

In the accompanying drawing I have used the reference numeral III toindicate a switch hous- 15 ing. The housing I0 has a hub l2 extendingtherefrom in which is rotatably mounted a shaft 14. A bimetal coil I6has one end secured as at I8 to the hub l2, and its other end secured asat 20 to the shaft l4. The elements l2 and [6 are 20 adapted forinsertion in the bonnet of a furnace or the rod l4 can be associated ina well known manner with a temperature responsive device, responsive tothe temperature of water or steam pressure in a hot water or steamheating system. 2

Within the switch housing l0 and mounted on posts 22 is a base 24,preferably of insulating material. The base 24 is held in position onthe post by screws 26. Upstanding from the base 24 is a bracket 28 towhich is secured a leaf spring 30. so

A strip of insulation 32 is secured to the leaf spring, and an armature34 is secured to the strip of insulation.

The armature 34 carriesa contact 36 for coaction with a stationarycontact 38. shown a permanent magnet for the purpose of imparting snapaction to the armature 34. The spring 30 tends to maintain the contact36 engaged with the contact 38.

For closing the contacts 36 and 38 I provide a wire motor comprising astationary projection 42 from the base 24, a floating plate 44 havingend members 46 of insulating material, and a resistance wire 48 woundaround the end elements, as

more fully disclosed in my co-pending application 5 Serial Number638,114, filed October 17, 1932.

A link 58 is provided with a head 52 through I which the resistance wire48 extends. The link is connectedby a leaf spring 54 with the armature34 for operating it in accordance with 5 the energization of the wiremotor element 48.

After the contacts 36 and 38 are closed,'they may be opened either byde-energization of the wire 48 or by a lobe 56 on a cam 58 engaging aleaf spring 60 of an extension 62 of the arma- At 40 is 3 ture 34. Thespring 68 prevents undue strain on the parts of the switch, but is stifienough to overcome the tension of the spring 30 for opening the contacts34 and 36 even though the wire 48 by energization has permitted them toclose.

On the back of the base 24 I provide a safety cutout switch comprisingcontact springs 64 and 66 normally held engaged by a bimetal element"means of a screw 82 threaded in a relatively stifl extension 84 of theelement 12. The adjusting lever 14 has an extension 86 which extendsthrough a slot 88 in the base 24, so that it can be adjusted from thefront side of the base. This adjustment permits the safety switch to-beadjusted for either fast or slow operation as desired.

A heating element 90 is mounted adjacent the bimetal element 68 forwarping it upwardly when heated for a predetermined period of time. Thispermits the spring 64 to swing toward the left for separating thecontacts carried by the springs 64 and 66, so that the circuit is lockedout of operation. It can be manually reset after the trouble isremedied, however, by pushing inwardly on a pin 92 which permits thebimetal element 68, after it has cooled down, to spring again to theposition shown in Figure 3.

Mounted on the switch housing I is a transformer 94. It is provided forsupplying low voltage current for operating the resistance wires 48 and90. The electrical hook-up is shown in Figure 4 wherein a roomthermostat. isdesignated at 96, a motor at M, and a stack switch at 98.The circuit arrangement is specifically claimed in my co-pendingapplication Serial ,Number 18,745 filed April 29, 1935.

Practical operation the heater 90 out of circuit, so that the look-outswitch will not operate to open the circuit, which, however, it will doafter a time period if combustion does not occur, or which it will do ifafter combustion establishment, combustion fails and thereby causes theswitch 98 to open.

Upon the bonnet of the'fumace attaining a predetermined high degree ofheat, the cam 58 1 will open the contacts 36 and 38, so that the furnacedoes not get too hot when the room thermostat calls for heat over anextended period of time. When the room thermostat is satisfied, the lowvoltage circuit is broken through its contacts so that the heatingelement 49 is de-energized and will contract to the position shown inFigure 2 for opening the contacts 36 and 38.

By mounting all of the operating parts for the switch contacts 36 and 38in a single casing and using this casing then as a limit switch so itcan be installed in the usual position of a limit switch, I insure thatthe dealer in making the installation, makes it properly, whereas I haveoften experienced that dealers sometimes omit limit switches when theyare independent of the other controlling apparatus to cheapen theinstallation and thus make an installation which is 5 not altogethersafe. Such an arrangement also simplifies wiring, since it is merelynecessary to extend current supply wires to and from the housing l0, andthen to the motor M, the only additional wires needed being the lowtension wires a and b, extending to the room thermostat and combustionswitch respectively. These, of course, need not be installed in conduit.

The plate 44 is preferably made of the same material as the heatingelement 48, so that its coefficient of expansion at various temperaturesis the same. This compensates for any ambient temperature change, sinceexpansion of the element 48 is equaled by expansion of the element 44.This also provides for the same time period of operation when thecurrent is cut ofi, regardless of whether the wire motor is relativelycool because of energization and deenergization soon thereafter, orwhether it has been well heated by a prolonged period of energization.25

One advantage of using a wire motor instead of a relay for closing thecontacts 36 and 38 is that it takes the wire motor some ten seconds orso to respond sufliciently to open the contacts, this lag in operationpreventing undesired starts and stops of the burner mechanism caused byvibration of the room thermostat contacts. Also when energized the wiremotor does not vibrate as when alternating current is used on anelectro-magnet.

A warp switch can be used instead of a wire motor, but it is too slow inoperation as the heat generated must first be transferred to a bimetalelement or the like before it can respond to energization. With a wiremotor, however,. since the electric current goes direct through thewire, it will expand immediately, thus giving much quicker operation,which is satisfactory for relay purposes, whereas a warp switch wouldnot be satisfactory.

By arranging the switch so that the spring 39 tends to close it, and theresistance wire 48 when cold opens it against such tendency wheneverthelow voltage circuit is broken from. any

cause, the wire 48 will contract to its cool position, causing thecontacts 36 and 38 to be opened. On the other hand, if the wire motor isso damaged that it permits the spring 30 to close the switch, it willagain be opened by the limit control l6, thus avoiding any dangerouslong period of operation of the fuel burner.

Some changes may be made in the construction and arrangement of theparts of my device without departing from the real spirit and purpose ofmy invention, and it is my intention to cover by my claims, any modifiedforms of 851110, ture or use of mechanical equivalents, which may bereasonably included within their scope.

I claim asv my invention:

1. In a combination limit switch, a switch casing, a normally openburner switch therein, electro-responsive means therein for closing saidburner switch and a temperature responsive element extending from saidcasing and adapted for mounting in a furnace-bonnet or the like, saidtemperature responsive element being operativ'ely associated with saidswitch to open the same while closed by said electro-responsive meansupon a predetermined limit oi! temperature being attained in saidfurnace bonnet.

semsm 2. in a, combination limit switch, a switch ms mg, a burner switchtherein, means for biasing said burner switch to closed positien, aresistance wire normally preventing closure of said. burnerswitchagainst the action of said biasing means for permitting closing ofsaid burner switch by said biasing means upon said. resistence wirebeing stretched by energization there- 0:6 and a temperature responsiveelement extending from said casing and seemed for mounting in a fm'naceMemes or the like, said temrzpemture responsive element. being opera-Lively assoeiated with said. switch to open the same while permitted toassume a closed posi bion by energization sf said resisiaenee wire upon.a pzedetermined limit of temperature being at tained in said; furnacebonnet.

LAWRENCE M. PERSQNS.

